The invention relates to a sigma-delta modulator for converting an analog input signal into a digital output signal, comprising:
an input network comprising a first gain stage for providing an amplified input signal in response to the analog input signal;
means for providing a difference signal in response to a comparison of the amplified input signal with an amplified feedback signal;
means for filtering the difference signal and for providing a filtered difference signal;
means for sampling and quantizing the filtered difference signal and having an output for providing the digital output signal;
a feedback network comprising a digital-to-analog converter for converting the digital output signal to an analog feedback signal and a second gain stage for providing the amplified feedback signal in response to the analog feedback signal.
Such a sigma-delta modulator (SDM) is generally known and described in books such as: Rudy van de Plassche, xe2x80x9cIntegrated analog-to-digital and digital-to-analog convertersxe2x80x9d, Kluwer Academic Publishers, 1994, Chapter 11. Sigma-delta modulation is a technique in which an analog input signal is converted into a digital output signal having high resolution and low quantisation noise with the aid of oversampling by quantisation means having a low resolution and a high quantisation noise. The digital signal is reconverted to an analog feedback signal by means of a digital-to-analog converter having the same low resolution and is subtracted from the analog input signal in a subtracting stage. The difference between the two signals is filtered in an analog loop filter and applied to the quantisation means. The use of a sufficiently high loop gain for baseband frequencies of the analog signal achieves that in the digital output signal the quantisation noise within the baseband is low at the expense of a higher quantisation noise above this baseband. By means of digital filter techniques, however, noise above the baseband can be suppressed effectively, for example, by means of a decimating filter that converts the oversampled SDM digital output signal into a higher resolution (more bits) digital signal at the desired lower sampling rate.
FIG. 1 shows a block diagram of the SDM. The analog input signal X is amplified or buffered in an input network, the gain of which is represented by a first gain stage having a gain a. An analog feedback signal is subtracted from the input signal and the difference is filtered in a low-pass loopfilter G(f), sampled at a sampling rate fs and quantized by a quantizer Q, which may have a 1-bit or multi-bit resolution. In a feedback network the digital output signal Ys is reconverted to an analog feedback signal by means of a digital-to-analog converter DAC having the same resolution as the quantizer Q. The analog gain of the feedback network is represented as a second gain stage having a gain d. As will be shown hereinafter, for low frequency inputs (that is for frequencies much smaller than the sampling rate fs) the signal gain is a/d. The quantization noise is xe2x80x9cshapedxe2x80x9d by the inverse of the loop-filter characteristic G(f).
For some uses, for example in instrumentation, the exact gain a/d of the SDM is important. However, due to mismatch between the gains of gain stages a and d the overall gain is not accurate and may deviate from the desired value, and the gaines may contain certain offsets, which differ from each other, so that the difference signal may have offset and distortion.
It is an object of the invention to provide an SDM with improved accuracy. To this end, the sigma-delta modulator as defined in the opening paragraph is characterized in that the sigma-delta modulator further comprises:
means for regularly interchanging the first gain stage and the second gain stage.
By regularly interchanging (xe2x80x9cchoppingxe2x80x9d) the first and second gain stages a and d the differences and further mismatches between both gain stages are modulated on the chopping frequency. The effective value of both gain stages becomes the average value of a and d so that the gain of the SDM is exactly unity. A high frequency ripple, at the chopping frequency, is present in the difference signal but the chopping frequency may be chosen outside the frequency band of interest, so that this ripple is of no importance.
Further improvements are obtained by using fully differential circuitry, preferably by using differential transconductors in the first and second gain stages. The subtraction of the input signal and the feedback signal can then easily be effected by interconnecting the opposite differential output signals of the transconductors.